Non-orthogonal plane-marching parabolized stability equations for the secondary instability of crossflow vortices
K.J.Groot, J. Casacuberta, S. Hickel (2026)
Computers & Fluids 306: 106947. doi: 10.1016/j.compfluid.2025.106947
A detailed derivation, analysis, and verification is given for the non-orthogonal, plane-marching Parabolized Stability Equations (PSE) approach. We show that converged solutions can be achieved for a broad frequency range with an existing stabilization method for the line-marching PSE approach. In applying the approach to a flow distorted by a medium-amplitude crossflow vortex, we determine its linear secondary instability mechanisms.
Baroclinic shift of hypersonic heating streaks over an ablating compression ramp
A.O. Başkaya, S. Hickel (2025)
Physics of Fluids 37: 116108. doi: 10.1063/5.0281627
We present novel observations from direct numerical simulations of transitional Mach 8 flow over a 15° compression ramp ablator. Heating streaks over the ramp are seen to undergo a half-wavelength shift near the location of transition from laminar to turbulent boundary layer flow. This phenomenon leads to an intriguing pattern of ablation grooves on the surface. Our analysis shows that the underlying mechanism is driven by the baroclinic torque in the strongly stratified near-wall region. We discuss the impact of this baroclinic shift for a surface undergoing ablative recession and assess its sensitivity to different thermal boundary conditions and perturbation amplitudes.
Streaky perturbations in swept-wing flow over forward-facing step
J. Casacuberta, S. Westerbeek, J.A. Franco, K.J. Groot, S. Hickel, S. Hein, M. Kotsonis (2025)
Physical Review Fluids 10: 023902. doi: 10.1103/PhysRevFluids.10.023902
Direct numerical simulations have been performed to scrutinize the stationary velocity-perturbation streaks that form close downstream of two-dimensional forward-facing step of fixed height embedded in laminar incompressible swept-wing boundary-layer flow.
Low-frequency dynamics of turbulent recirculation bubbles
L. Laguarda, S. Hickel (2024)
Physics of Fluids 36: 081708. doi: 10.1063/5.0227332
We revisit the origin of low-frequency unsteadiness in turbulent recirculation bubbles (TRBs), and, in particular, the hypothesis of a dynamic feedback mechanism between unconstrained separation and reattachment locations. Our results demonstrate, for the first time, effective suppression of the low-frequency characteristics of the TRB without reducing its size, strongly supporting our hypothesis.
Passive Control of Shock-Wave/Turbulent Boundary-Layer Interaction Using Spanwise Heterogeneous Roughness
W. Wu, L. Laguarda, D. Modesti, S. Hickel (2025)
Flow, Turbulence and Combustion 115: 29-49. doi: 10.1007/s10494-024-00580-0
A novel passive flow-control method for shock-wave/turbulent boundary-layer interactions (STBLI) is investigated. The method relies on a structured roughness pattern constituted by streamwise-aligned ridges. Its effectiveness is assessed with wall-resolved large-eddy simulations of the interaction of a Mach 2 turbulent boundary layer flow with an oblique impinging shock with shock angle 40°. A parametric study is performed to investigate the effect of the spacing between the ridges. We find that ridges with small spacing effectively mitigate the low-frequency unsteadiness of STBLI and slightly reduce total-pressure loss.
Passive stabilization of crossflow instabilities by a reverse lift-up effect
J. Casacuberta, S. Hickel, M. Kotsonis (2024)
Physical Review Fluids 9: 043903. doi: 10.1103/PhysRevFluids.9.043903
A novel mechanism is identified, through which a spanwise-invariant surface feature (a two-dimensional forward-facing step) significantly stabilizes the stationary crossflow instability of a three-dimensional boundary layer. The mechanism is termed here as reverse lift-up effect, inasmuch as it acts reversely to the classic lift-up effect; that is, kinetic energy of an already-existing shear-flow instability is transferred to the underlying laminar flow through the action of cross-stream perturbations.
Convective instabilities in a laminar shock-wave/boundary-layer interaction
S.E.M. Niessen, K.J. Groot, S. Hickel, V.E. Terrapon (2023)
Physics of Fluids 35: 024101. doi: 10.1063/5.0135590
Linear stability analyses are performed to study the dynamics of linear convective instability mechanisms in a laminar shock-wave/boundary-layer interaction at Mach 1.7. In order to account for all two-dimensional gradients elliptically, we introduce perturbations into an initial-value problem that are found as solutions to an eigenvalue problem formulated in a moving frame of reference.
Experimental and numerical investigation into the drag performance of dimpled surfaces in a turbulent boundary layer
O.W.G. van Campenhout, M. van Nesselrooij, Y.Y. Lin, B.W. van Oudheusden, S. Hickel (2023)
International Journal of Heat and Fluid Flow 100: 109110. doi: 10.1016/j.ijheatfluidflow.2023.109110
Although several previous studies have reported a potential drag-reducing effect of dimpled surfaces in turbulent boundary layers, there is a lack of replicability across experiments performed by different research groups. To contribute to the dialogue, we scrutinize one of the most studied dimple geometries reported in the literature, which has a dimple diameter of 20 mm and a depth of 0.5 mm.
Unsteady mechanisms in shock wave and boundary layer interactions over a forward-facing step
W. Hu, S. Hickel, B.W. van Oudheusden (2022)
Journal of Fluid Mechanics 949: A2. doi: 10.1017/jfm.2022.737
The flow over a forward-facing step (FFS) at Ma∞=1.7 and Re? = 13 718 is investigated by well-resolved large-eddy simulation. To investigate effects of upstream flow structures and turbulence on the low-frequency dynamics of the shock wave/boundary layer interaction (SWBLI), two cases are considered: one with a laminar inflow and one with a turbulent inflow.
Direct numerical simulation of interaction between a stationary crossflow instability and forward-facing steps
J. Casacuberta, S. Hickel, S. Westerbeek, M. Kotsonis (2022)
Journal of Fluid Mechanics 943: A46. doi: 10.1017/jfm.2022.456
The interaction between forward-facing steps of several heights and a pre-existing critical stationary crossflow instability of a swept-wing boundary layer is analysed.
Secondary instabilities in swept-wing boundary layers: Direct Numerical Simulations and BiGlobal stability analysis
J. Casacuberta, K.J. Groot, S. Hickel, M. Kotsonis (2022)
SciTech Forum and Exposition, San Diego. AIAA paper 2022-2330, doi: 10.2514/6.2022-2330
The evolution of secondary instabilities in a three-dimensional stationary-crossflow-dominated boundary layer is investigated by means of Direct Numerical Simulations (DNS) and linear spanwise BiGlobal stability analysis. Single-frequency unsteady disturbances and a critical stationary crossflow mode are considered.
Mechanisms of interaction between stationary crossflow instabilities and forward-facing steps
J. Casacuberta, S. Hickel, M. Kotsonis (2021)
AIAA Scitech paper 2021-0854. doi: 10.2514/6.2021-0854
We study the interaction between a stationary crossflow instability and forward-facing steps in a swept-wing boundary layer using Direct Numerical Simulations (DNS). The station- ary primary crossflow mode is imposed at the inflow. Steps of several heights are modeled.
Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step
W. Hu, S. Hickel, B.W. van Oudheusden (2021)
Journal of Fluid Mechanics 915: A107. doi: 10.1017/jfm.2021.95
The low-frequency unsteady motions behind a backward-facing step (BFS) in a turbulent flow at Ma=1.7 and Re∞=1.3718×107 m−1 are investigated using a well-resolved large-eddy simulation.
Inertia gravity waves breaking in the middle atmosphere: energy transfer and dissipation tensor anisotropy
T. Pestana, M. Thalhammer, S. Hickel (2020)
Journal of the Atmospheric Sciences 77: 3193-3210. doi: 10.1175/JAS-D-19-0342.1
We present direct numerical simulations of inertia–gravity waves breaking in the middle–upper mesosphere. We consider two different altitudes, which correspond to the Reynolds number of 28 647 and 114 591 based on wavelength and buoyancy period. While the former was studied by Remmler et al., it is here repeated at a higher resolution and serves as a baseline for comparison with the high-Reynolds-number case.
Influence of upstream disturbances on the primary and secondary instabilities in a supersonic separated flow over a backward-facing step
W. Hu, S. Hickel, B.W. van Oudheusden (2020)
Phys. Fluids 32: 056102. doi: 10.1063/5.0005431
Dynamics of unsteady asymmetric shock interactions
L. Laguarda, S. Hickel, F.F.J. Schrijer, B.W. van Oudheusden (2020)
Journal of Fluid Mechanics 888: A18. doi: 10.1017/jfm.2020.28
The response of asymmetric and planar shock interactions to a continuous excitation of the lower incident shock is investigated numerically. Incident shock waves and centred expansion fans are generated by two wedges asymmetrically deflecting the inviscid free stream flow at Mach 3.